A method for transient frequency modulation control of wind turbine

Abstract

The invention discloses a method for controlling transient frequency regulation of a wind turbine, comprising: obtaining the relationship between the frequency threshold of a wind turbine grid-connected system and a virtual inertia control parameter, and obtaining a critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system; obtaining a critical frequency trajectory curve that meets the frequency requirement of the wind turbine grid-connected system and contains unknown parameters according to the critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system; and obtaining a critical frequency trajectory with known parameters according to the relationship between the frequency threshold of the wind turbine grid-connected system and a relay protection action threshold that meets the requirements of power grid guidelines; using the critical frequency trajectory with known parameters as a frequency control target of the wind turbine grid-connected system to complete transient frequency regulation control of the wind turbine grid-connected system; the invention makes full use of the characteristics of flexible output power control and strong plasticity of a wind turbine power electronic device; and realizes the transient frequency support of the wind turbine and ensures the economy of wind turbine power generation.

Description

Technical Field

[0001] The invention belongs to the technical field of wind power frequency modulation control, and in particular relates to a wind turbine transient frequency modulation control method. Background Art

[0002] With the large-scale access of wind turbines using power electronic equipment as grid-connected interfaces to the power grid, the traditional power grid has gradually transformed into a new power system with low inertia and weak damping dominated by new energy, which has completely changed the dynamic characteristics and stability characteristics of the traditional power grid. In traditional power grids, synchronous generators with large inertia and strong damping as typical characteristics have natural frequency response capabilities and can autonomously adjust output power according to system frequency changes; therefore, the frequency offset (Δf) and the rate of change of frequency (RoCoF) indicators can easily meet the requirements of power grid guidelines; however, unlike traditional power grids, for new power systems with high wind power penetration, wind turbines convert and transmit energy with large power grids through power electronic equipment. Due to the decoupling effect of power electronic equipment, the ability of the power grid to suppress random disturbances and maintain frequency stability is weakened; at the same time, wind power generation has low-density dispersion on a spatial scale and strong random volatility on a time scale. Large-scale access of wind turbines has exacerbated the randomness and volatility of the power generation side; under the dual background of increased random disturbances and reduced anti-disturbance capabilities, high wind power penetration power grids face severe frequency stability issues; in order to cope with the reduced system inertia and insufficient frequency regulation capability brought about by high wind power penetration, the active participation of wind turbines in grid frequency regulation has become an inevitable choice after large-scale grid connection of wind power.

[0003] Existing frequency regulation strategies for wind turbines without backup power mainly include virtual inertia control (VIC), virtual synchronous machine and generalized droop control, etc.; the above control strategies mainly use the rotating mass of the wind turbine rotor or supercapacitor as a virtual inertia source, and their advantage is that the wind turbine does not need to reserve frequency regulation backup power in advance; the basic principle of the above frequency regulation strategy is to enable the wind turbine to provide additional frequency regulation power proportional to the frequency offset Δf or the frequency change rate RoCoF, which is essentially equivalent to the classic droop control method, inertia control method and proportional differential (PD) control method; in theory, increasing the controller gain of the wind turbine grid-connected inverter can provide more frequency regulation power and further suppress the frequency offset Δf or the frequency change rate RoCoF. However, most of the design capacity of wind turbine grid-connected inverters has been used to undertake the core function of power transmission, and the available resources for frequency regulation are very limited. At the same time, wind turbines using fixed controller gains cannot adaptively output the optimal frequency regulation power according to wind speed changes and the intensity of random disturbances faced by the system, and therefore cannot efficiently utilize the limited available frequency regulation resources. In addition, the strength of the frequency regulation response capability of wind turbines is affected by the operating status of the wind turbine, comprehensive control parameters, the degree of system disturbance and system parameters. However, due to the randomness of the disturbance and the volatility of wind speed, it is difficult to determine the appropriate control parameters, and thus it is impossible to guarantee a wind turbine frequency regulation response with high reliability. Summary of the invention

[0004] In view of the technical problems existing in the prior art, the present invention provides a transient frequency modulation control method for a wind turbine to solve the problem that in the existing wind turbine frequency modulation control strategy, the gain parameters of the controller are easily affected by unknown grid parameters such as grid inertia, damping coefficient and disturbance, and different operating conditions require different control parameters; at the same time, due to the randomness of the disturbance and the volatility of the wind speed, it is difficult to determine the appropriate controller control parameters, and thus it is impossible to ensure a high-reliability wind turbine frequency modulation response, which greatly reduces the technical problem of the economic efficiency of wind turbine grid connection.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] The present invention provides a method for controlling transient frequency modulation of a wind turbine, comprising:

[0007] Based on the frequency dynamic response model of the wind turbine grid-connected system, the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameters is obtained;

[0008] According to the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter, a critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is obtained;

[0009] According to the critical virtual inertia control parameters for ensuring the frequency safety of the wind turbine grid-connected system, a critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters is obtained;

[0010] Based on the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters, and according to the relationship between the frequency threshold of the wind turbine grid-connected system and the relay protection action threshold that meets the requirements of the power grid guidelines, a critical frequency trajectory with known parameters is obtained;

[0011] When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, the critical frequency trajectory with known parameters is used as the frequency control target of the wind turbine grid-connected system to complete transient frequency modulation control of the wind turbine grid-connected system.

[0012] Furthermore, the frequency dynamic response model of the wind turbine grid-connected system is:

[0013]

[0014] Where Δf(s) is the frequency deviation in the frequency domain after the wind turbine grid-connected system is disturbed; s is the complex frequency; ΔP D is the step disturbance power; K L,f is the equivalent damping coefficient of the power grid; T JS is the equivalent inertia constant of the power grid; K d K is the inertia gain coefficient in virtual inertia control; p is the droop damping coefficient in virtual inertia control.

[0015] Furthermore, the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter is:

[0016]

[0017] Wherein, Δf(t) is the frequency deviation of the wind turbine grid-connected system in the time domain; t is time; RoCoF is the frequency change rate of the wind turbine grid-connected system.

[0018] Furthermore, the critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is:

[0019]

[0020] Among them, K pcr To ensure the minimum value of the droop damping coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety; K dcr To ensure the minimum value of the inertia gain coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety; F thr is the frequency deviation relay protection action threshold required by the grid guidelines; R thrIt is the frequency change rate relay protection action threshold required by the power grid guidelines.

[0021] Furthermore, the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters is:

[0022]

[0023] Among them, f TP_unknown (s) is the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters; f N (s) is the rated frequency of the wind turbine grid-connected system.

[0024] Furthermore, the critical frequency trajectory of the known parameters is:

[0025]

[0026] Among them, f TP_known (s) is the critical frequency trajectory with known parameters.

[0027] Further, when the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, the critical frequency trajectory with known parameters is used as the frequency control target of the wind turbine grid-connected system to complete the transient frequency modulation control process of the wind turbine grid-connected system, as follows:

[0028] When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, a general expression of the critical frequency trajectory is obtained according to the critical frequency trajectory with known parameters;

[0029] According to the general expression of the critical frequency trajectory, the PD control method is adopted to keep the frequency response curve of the wind turbine grid-connected system within the safety margin range of the critical frequency trajectory with known parameters, thereby completing the transient frequency regulation control of the wind turbine grid-connected system.

[0030] Furthermore, the general expression of the critical frequency trajectory is:

[0031]

[0032]

[0033] R=df / dt

[0034] Among them, f TP (s) is the critical frequency trajectory; f is the actual frequency of the wind turbine grid-connected system; R is the actual frequency change rate of the wind turbine grid-connected system; sign(*) is the sign function.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention provides a method for controlling transient frequency modulation of a wind turbine. Based on a frequency dynamic response model of a wind turbine grid-connected system, a critical frequency trajectory with a safety margin and known parameters is actively planned according to the frequency threshold of the wind turbine grid-connected system; the frequency modulation power of the wind turbine is regulated by the critical frequency trajectory with known parameters to achieve rapid support for the transient frequency of the wind turbine grid-connected system; wherein, the critical frequency trajectory with known parameters is used as a control target, so that the wind turbine provides transient frequency support strictly in accordance with the pre-planned frequency trajectory, and compared with the traditional virtual inertia control method, the method has better economy when achieving transient frequency support of the wind turbine; at the same time, it can effectively avoid a deep drop in the speed of the wind turbine; secondly, it can effectively avoid complex control parameter design, and is not affected by system inertia, damping and random interference; the present invention can make full use of the characteristics of flexible output power control and strong plasticity of the wind turbine power electronic device, and under the background of limited frequency modulation resources, it not only achieves transient frequency support of the wind turbine, but also ensures the economy of wind turbine power generation. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a principle block diagram of a wind turbine grid-connected system model in an embodiment;

[0038] Figure 2 It is a schematic diagram of the frequency trajectory planning principle in the embodiment;

[0039] Figure 3 is the frequency modulation power P in the embodiment cut Schematic diagram of the change with the disturbance coefficient δ;

[0040] Figure 4 A control block diagram for implementing frequency trajectory planning in an embodiment;

[0041] Figure 5 It is a schematic diagram of the frequency trajectory partition in the embodiment; wherein, Figure 5 (a) is a schematic diagram of the frequency trajectory partition after the frequency deviation Δf is triggered; Figure 5 (b) is a schematic diagram of the frequency trajectory partition after the frequency change rate RoCoF is triggered; Figure 5 (c) is the frequency deviation Δf variation curve; Figure 5 (d) is the frequency change rate RoCoF change curve;

[0042] Figure 6 is a control model diagram of a wind turbine single-machine grid-connected system in an embodiment;

[0043] Figure 7 : is a test result diagram of the droop damping coefficient in different virtual inertia controls in the embodiment; wherein, Figure 7 (a) is the frequency curve; Figure 7 (b) is the frequency modulation power curve; Figure 7 (c) is the frequency curve of TP and FTP; Figure 7 (d) is the rotor speed curve of the fan;

[0044] Figure 8 is the frequency modulation electromagnetic torque T of the system under the droop damping coefficient conditions in different virtual inertia control in the embodiment e and mechanical torque T w The test result diagram of Figure 8 (a) is the frequency modulation electromagnetic torque T of the system under the condition of droop damping coefficient of 50e6 e and mechanical torque T w The test result diagram of Figure 8 (b) is the frequency modulation electromagnetic torque T of the system under the condition of droop damping coefficient of 80e6 e and mechanical torque T w The test result diagram of Figure 8 (c) is the frequency modulation electromagnetic torque T of the system under the condition that the droop damping coefficient is 10e6 e and mechanical torque T w The test result diagram of Figure 8 (d) is the frequency modulation electromagnetic torque T of the system under frequency trajectory planning control e and mechanical torque T w The test result diagram of

[0045] Fig. 9 is a test result diagram at low wind speed in the embodiment; wherein, Fig. 9 (a) is the frequency curve; Fig. 9 (b) is the frequency modulation power curve; Fig. 9 (c) is the frequency curve of TP and FTP; Fig. 9 (d) is the rotor speed curve of the fan; Fig. 9 (e) is the frequency modulation electromagnetic torque T of the system under frequency trajectory planning control e and mechanical torque T w The test result diagram of Fig. 9 (f) is the frequency modulation electromagnetic torque T of the system under virtual inertia control e and mechanical torque T w The test result diagram of

[0046] Fig.10 is the experimental result under medium wind speed in the embodiment; wherein, Fig.10 (a) is the frequency curve; Fig.10 (b) is the frequency modulation power curve; Fig.10 (c) is the frequency curve of TP and FTP; Fig.10 (d) is the rotor speed curve of the fan; Fig.10 (e) is the frequency modulation electromagnetic torque T of the system under frequency trajectory planning controle and mechanical torque T w The test result diagram of Fig.10 (f) is the frequency modulation electromagnetic torque T of the system under virtual inertia control e and mechanical torque T w The test result diagram of

[0047] Fig.11 is the experimental result under high wind speed in the embodiment; wherein, Fig.11 (a) is the frequency curve; Fig.11 (b) is the frequency modulation power curve; Fig.11 (c) is the frequency curve of TP and FTP; Fig.11 (d) is the rotor speed curve of the fan; Fig.11 (e) is the frequency modulation electromagnetic torque T of the system under frequency trajectory planning control e and mechanical torque T w The test result diagram of Fig.11 (f) is the frequency modulation electromagnetic torque T of the system under virtual inertia control e and mechanical torque T w The test result diagram of

[0048] Fig.12 The IEEE standard four-machine two-area model diagram including wind turbines in the embodiment;

[0049] Fig.13 The experimental results of the four-machine two-zone system in the embodiment; wherein, Fig.13 (a) Frequency curve; Fig.13 (b) Actual frequency planning curve; Fig.13 (c) FM auxiliary power curve. DETAILED DESCRIPTION

[0050] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail in the following specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0051] The present invention provides a method for transient frequency modulation control of a wind turbine, comprising the following steps:

[0052] Step 1: Based on the frequency dynamic response model of the wind turbine grid-connected system, the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter is obtained; wherein the frequency dynamic response model of the wind turbine grid-connected system is:

[0053]

[0054] Where Δf(s) is the frequency deviation in the frequency domain after the wind turbine grid-connected system is disturbed; s is the complex frequency; ΔP D is the step disturbance power; K L,f is the equivalent damping coefficient of the power grid; T JS is the equivalent inertia constant of the power grid; K d K is the inertia gain coefficient in virtual inertia control; p is the droop damping coefficient in virtual inertia control.

[0055] In step 1, the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter is the frequency threshold including the virtual inertia control parameter obtained through the frequency dynamic response model of the wind turbine grid-connected system; wherein the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter is:

[0056]

[0057] Among them, Δf(t) is the frequency deviation of the wind turbine grid-connected system in the time domain; t is time; RoCoF is the frequency change rate of the power grid-connected system.

[0058] Step 2: According to the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter, a critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is obtained; wherein the critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is:

[0059]

[0060] Among them, K pcr To ensure the minimum value of the droop damping coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety; K dcr To ensure the minimum value of the inertia gain coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety; F thr is the frequency deviation relay protection action threshold required by the grid guidelines; R thr It is the frequency change rate relay protection action threshold required by the power grid guidelines.

[0061] Step 3: According to the critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system, a critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and includes unknown parameters is obtained; wherein the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and includes unknown parameters is:

[0062]

[0063] Among them, f TP_unknown (s) is the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters; fN (s) is the rated frequency.

[0064] In the present invention, there are many unknown parameters in the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system, for example; the grid equivalent damping coefficient K L,f , grid equivalent inertia constant T JS , step disturbance power ΔP D , ensure the minimum value K of the droop damping coefficient in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system pcr And the minimum value K of the inertia gain coefficient in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system dcr Therefore, the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters cannot be directly used in actual frequency regulation control.

[0065] Step 4: Based on the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters, and according to the relationship between the frequency threshold of the wind turbine grid-connected system and the relay protection action threshold that meets the grid guidelines, obtain the critical frequency trajectory with known parameters; wherein the critical frequency trajectory with known parameters is:

[0066]

[0067] Among them, f TP_known (s) is the critical frequency trajectory with known parameters.

[0068] Step 5: When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, the critical frequency trajectory with known parameters is used as the frequency control target of the wind turbine grid-connected system to complete the transient frequency modulation control of the wind turbine grid-connected system; wherein the specific process is as follows:

[0069] When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, a general expression of the critical frequency trajectory is obtained according to the critical frequency trajectory with known parameters;

[0070] Among them, the general expression of the critical frequency trajectory is:

[0071]

[0072]

[0073] R=df / dt

[0074] Among them, f TP (s) is the critical frequency trajectory; f is the actual frequency of the wind turbine grid-connected system; R is the actual frequency change rate of the wind turbine grid-connected system; sign (*) is the sign function;

[0075] According to the general expression of the critical frequency trajectory, the PD control method is adopted to keep the frequency response curve of the wind turbine grid-connected system within the safety margin range of the critical frequency trajectory with known parameters, thereby completing the transient frequency regulation control of the wind turbine grid-connected system.

[0076] In the present invention, the parameters in the general expression of the critical frequency trajectory are all frequency thresholds specified by the grid guidelines, or system operating parameters that can be measured online in real time, which are independent of load disturbances and system parameters; therefore, the general expression of the critical frequency trajectory can be directly used in actual frequency regulation control.

[0077] The transient frequency modulation control method for wind turbines described in the present invention is essentially that after the power system is disturbed, the control target of the wind turbine is switched from the rated frequency to the planned frequency trajectory; the wind turbine only provides the frequency modulation power required to ensure that the frequency deviation Δf and the frequency change rate RoCoF do not exceed their frequency thresholds; if the natural frequency response of the frequency after the disturbance is within the safe area, the wind turbine does not need to provide transient frequency support; thereby, while ensuring the safe and stable operation of the system frequency, the economic benefits of wind turbine grid-connected power generation are improved; if the wind turbine operates strictly according to the planned frequency trajectory, it can not only meet the requirements of the power grid operation for frequency indicators, but also ensure the economic benefits of wind turbine grid-connected power generation; among them, the control method for the wind turbine to operate strictly according to the planned frequency trajectory is called frequency trajectory planning (FTP) control method.

[0078] Example

[0079] This embodiment provides a method for controlling transient frequency modulation of a wind turbine, comprising the following steps:

[0080] S1. Frequency dynamic response analysis of wind turbine grid-connected system

[0081] Establish an equivalent model of the wind turbine grid-connected system; Figure 1 As shown, attached Figure 1 The principle block diagram of the wind turbine grid-connected system model is given in the figure; according to the equivalent model of the wind turbine grid-connected system, the frequency model of the wind turbine grid-connected system is determined; wherein, the frequency model of the wind turbine grid-connected system is determined by the output power P of the power grid. G Load power P L And the fan output power P W Working together;

[0082] Right now:

[0083]

[0084] Among them, T JSis the equivalent inertia constant of the power grid, and f is the power grid frequency of the wind turbine grid-connected system.

[0085] According to the frequency model of the wind turbine grid-connected system, a frequency dynamic response model of the wind turbine grid-connected system is constructed; wherein the frequency dynamic response model of the wind turbine grid-connected system is:

[0086]

[0087] Where Δf(s) is the frequency deviation in the frequency domain after the wind turbine grid-connected system is disturbed; s is the complex frequency; ΔP D is the step disturbance power, ΔP D >0;K L,f is the equivalent damping coefficient of the power grid; T JS is the equivalent inertia constant of the power grid; K d is the inertia gain coefficient in virtual inertia control; K p is the droop damping coefficient in virtual inertia control.

[0088] Based on the frequency dynamic response model of the wind turbine grid-connected system, the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter is obtained; specifically, by solving the above formula (2), the frequency threshold of the wind turbine grid-connected system can be obtained; wherein the frequency threshold of the wind turbine grid-connected system includes the maximum frequency deviation of the wind turbine grid-connected system and the maximum frequency change rate of the wind turbine grid-connected system.

[0089] The frequency threshold of the wind turbine grid-connected system, that is, the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter is:

[0090]

[0091] Wherein, Δf(t) is the frequency deviation of the wind turbine grid-connected system in the time domain; t is time; RoCoF is the frequency change rate of the wind turbine grid-connected system.

[0092] From the analysis of formula (3), it can be seen that the occurrence of disturbance will inevitably cause the grid frequency of the wind turbine grid-connected system to change, which will induce the frequency deviation Δf and the frequency change rate RoCoF to exceed the limit; if the frequency deviation Δf and the frequency change rate RoCoF exceed the limit, the frequency-related protection will be triggered; at the same time, formula (3) also shows that: under the same disturbance, the inertia gain coefficient K in the virtual inertia control d and the droop damping coefficient K in virtual inertia control p The smaller it is, the easier it is for the frequency deviation Δf and the frequency change rate RoCoF to exceed the limit; in order to reduce the maximum value of the frequency deviation Δf and the frequency change rate RoCoF, the wind turbine power generation system cannot always operate in the MPPT mode, and sufficient frequency regulation power needs to be provided to actively support the frequency of the low inertia power system.

[0093] Under virtual inertia control (VIC), the auxiliary frequency modulation power of the fan is:

[0094]

[0095] Among them, P vic (s) is the auxiliary frequency modulation power of the wind turbine in the frequency domain.

[0096] According to the auxiliary frequency modulation power of the fan under the virtual inertia control (VIC), the maximum auxiliary frequency modulation power provided by the virtual inertia control strategy is obtained as follows:

[0097]

[0098] Among them, P vic|max The maximum auxiliary frequency modulation power provided by the virtual inertia control strategy; P vic (t) is the auxiliary frequency modulation power of the wind turbine in the time domain; P vic (s) are the auxiliary frequency regulation powers of the wind turbines in the frequency domain.

[0099] In this embodiment, the auxiliary frequency modulation power of the fan under the virtual inertia control is adjusted along with the inertia gain coefficient K in the virtual inertia control. d and the droop damping coefficient K in virtual inertia control p Increases and becomes larger; the inertia gain coefficient K in the larger virtual inertia control d and the droop damping coefficient K in virtual inertia control p It can ensure more frequency modulation output of the wind turbine and improve the system frequency quality, but it will inevitably pay an economic price for this: the extreme deviation from the MPPT working point and the deep reduction of the wind turbine rotor speed will make the wind turbine unable to cope with continuous disturbances; in contrast, the inertia gain coefficient K in the smaller virtual inertia control d and the droop damping coefficient K in virtual inertia control p Although the depth of the fan rotor speed drop can be reduced, it may not be able to effectively cope with external disturbances. When the disturbance is large, the risk of system frequency exceeding the limit will be significantly increased. Therefore, in real massive operating scenarios, it is difficult to achieve the best frequency regulation effect for each specific working condition.

[0100] S2. Frequency trajectory planning control

[0101] Frequency is an important indicator of the power quality of wind turbine grid-connected systems. The maximum value of the frequency deviation Δf and the maximum value of the frequency change rate RoCoF in the wind turbine grid-connected system cannot exceed the relay protection action threshold specified in the grid guidelines; that is:

[0102]

[0103] Among them, F thr is the frequency deviation relay protection action threshold required by the grid guidelines; R thr It is the frequency change rate relay protection action threshold required by the power grid guidelines.

[0104] According to the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter, the critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is obtained; specifically, by solving formula (6), the virtual inertia control parameter that meets the requirements of the power grid guideline is obtained, that is, the critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is obtained.

[0105] Among them, the critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is:

[0106]

[0107] Among them, K pcr To ensure the minimum value of the droop damping coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety; K dcr To ensure the minimum value of the inertia gain coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety.

[0108] In this embodiment, under the premise of ensuring that the frequency-related relay protection action is not triggered, the wind turbine is prevented from deviating from the maximum power point when participating in frequency regulation to ensure the economic efficiency of power generation of the wind turbine grid-connected system; at the same time, in order to retain as much available frequency regulation capacity as possible to cope with subsequent interference, the optimal wind turbine frequency regulation is to minimize the maximum auxiliary frequency regulation power provided by the virtual inertia control strategy shown in formula (5) under the constraints shown in formula (6) and formula (7).

[0109] Therefore, when the virtual inertia control parameter is the minimum droop damping coefficient K in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system pcr , ensure the minimum value K of the inertia gain coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety dcr When , the optimal auxiliary power output of virtual inertia control can be achieved; however, the minimum droop damping coefficient K in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system given by formula (7) pcr , ensure the minimum value K of the inertia gain coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety dcr The expression contains the system's own inertia coefficient T JS , equivalent damping coefficient K L,f , external random disturbance ΔP D ,The above parameters are not only difficult to obtain in advance, but will also change with changes in system operation mode, load fluctuations, line faults, etc.

[0110] In order to ensure the stability of the grid frequency after a disturbance occurs, the virtual inertia control parameters can only be designed according to the worst possible operating conditions, that is, the maximum random disturbance and the minimum system inertia damping; therefore, under most operating conditions, virtual inertia control is difficult to provide just the right optimal inertia damping support.

[0111] From formula (7), it can be seen that when the control parameter is the minimum value K of the droop damping coefficient in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system pcr , ensure the minimum value K of the inertia gain coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety dcr When the frequency support provided by the virtual inertia control after the disturbance just meets the frequency requirement in formula (7); at this time, the frequency curve of the wind turbine grid-connected system is exactly the critical frequency trajectory that meets the grid frequency requirement; therefore, the minimum droop damping coefficient K in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system is pcr , ensure the minimum value K of the inertia gain coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety dcr Substituting into formula (2), we can obtain the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters; where the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters is:

[0112]

[0113] Among them, f TP_unknown (s) is the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters; f N (s) is the rated frequency of the wind turbine grid-connected system.

[0114] In the embodiment 1, the expression of the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters has many position parameters, such as the equivalent damping coefficient K of the power grid. L,f , grid equivalent inertia constant T JS , step disturbance power ΔP D , ensure the minimum value K of the droop damping coefficient in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system pcr And the minimum value K of the inertia gain coefficient in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system dcr ; This critical frequency trajectory expression is difficult to use in practice, so it needs to be further improved.

[0115] Based on the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters, and according to the relationship between the frequency threshold of the wind turbine grid-connected system and the relay protection action threshold that meets the requirements of the power grid guidelines, the critical frequency trajectory with known parameters is obtained; the specific process is as follows:

[0116] The frequency deviation relay protection action threshold F required by the known power grid code is used. thr And the frequency change rate relay protection action threshold R required by the power grid guidelines thr Rewrite the formula (6) to obtain formula (9);

[0117]

[0118] Substituting formula (9) into formula (8), we can obtain the critical frequency trajectory with known parameters. Figure 2 As shown, attached Figure 2 A schematic diagram of the frequency trajectory planning principle in the embodiment is given in FIG. Figure 2 The curve in the upper right corner is the critical frequency trajectory for which the parameters are known.

[0119] Among them, the critical frequency trajectory with known parameters is:

[0120]

[0121] Among them, f TP_known (s) is the critical frequency trajectory with known parameters.

[0122] When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, the critical frequency trajectory with known parameters is used as the frequency control target of the wind turbine grid-connected system to complete the transient frequency modulation control of the wind turbine grid-connected system; wherein the specific process is as follows:

[0123] When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory of the known parameters, that is, the step disturbance power ΔP D <0, according to the critical frequency trajectory with known parameters, a general expression of the critical frequency trajectory is obtained;

[0124] Among them, the general expression of the critical frequency trajectory is:

[0125]

[0126]

[0127] R=df / dt (13)

[0128] Among them, f TP(s) is the critical frequency trajectory; f is the actual frequency of the wind turbine grid-connected system; R is the actual frequency change rate of the wind turbine grid-connected system; sign (*) is the sign function;

[0129] According to the general expression of the critical frequency trajectory, the PD control method is adopted to keep the frequency response curve of the wind turbine grid-connected system within the safety margin range of the critical frequency trajectory with known parameters, thereby completing the transient frequency regulation control of the wind turbine grid-connected system.

[0130] S3. Economic comparison between the frequency trajectory planning FTP in this embodiment and the existing virtual inertia control VIC

[0131] In this embodiment, when the wind turbine grid-connected system operates strictly according to the general expression of the critical frequency trajectory, that is, the frequency trajectory planned by formula (11), the frequency modulation auxiliary power of the wind turbine during disturbance is obtained as follows:

[0132]

[0133] In order to replace the unknown minimum value K of the droop damping coefficient in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system pcr , ensure the minimum value K of the inertia gain coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety dcr , Substituting formula (9) into formula (14), we can get the FM auxiliary power with known parameters;

[0134] Among them, the frequency auxiliary power with known parameters is:

[0135]

[0136] The frequency modulation auxiliary power of the fan under virtual inertia control VIC is affected by the droop damping coefficient K in the virtual inertia control. p , Inertia gain coefficient K in virtual inertia control d and is constrained by formula (7); the design is performed under the worst operating conditions according to the virtual inertia control VIC parameters, assuming that the maximum disturbance that the wind turbine grid-connected system may be subjected to is the maximum step disturbance power ΔP D_max , then the droop damping coefficient K in virtual inertia control p , Inertia gain coefficient K in virtual inertia control d It can be written as:

[0137]

[0138] Define the actual step disturbance power ΔP D and the maximum step disturbance power ΔP D_max The ratio of is the disturbance intensity coefficient δ, that is:

[0139]

[0140] When the actual step disturbance power received by the wind turbine grid-connected system exceeds the maximum step disturbance power, that is, δ>1, it means that the frequency modulation auxiliary power output of the wind turbine has reached its limit, and the system frequency-related protection will be activated; at this time, the frequency safety of the wind turbine grid-connected system can no longer be effectively guaranteed, and it is meaningless to discuss the economic feasibility of wind turbine frequency modulation; therefore, the disturbance in the subsequent analysis is by default within the maximum disturbance range, that is: 0<δ<1.

[0141] Substituting formula (17) into formula (16), we can obtain the droop damping coefficient K in the virtual inertia control under maximum disturbance: p , Inertia gain coefficient K in virtual inertia control d for:

[0142]

[0143] Substituting formula (18) into formula (4), we can obtain the frequency modulation auxiliary power P provided by the wind turbine when using virtual inertia control VIC: vic for:

[0144]

[0145] Comparing formula (15) with formula (19), the frequency modulation power P saved by the fan using frequency trajectory planning control FTP is obtained compared with using virtual inertia control VIC. cut for:

[0146]

[0147] As attached Figure 3 As shown, attached Figure 3 The frequency modulation power P in the embodiment is given in cut Schematic diagram of the change of disturbance coefficient δ; Figure 3 It can be seen from the figure that the economic advantage of frequency trajectory planning to control FTP gradually increases with the increase of transient frequency support time and the decrease of disturbance intensity δ; in particular, most of the disturbances faced by the wind turbine grid-connected system are random small disturbances, that is, δ is small; therefore, the frequency trajectory planning to control FTP described in this embodiment has obvious economic advantages under most disturbance conditions; wherein, the frequency modulation power P cut The maximum value of the equivalent damping coefficient K of the power grid L,f , grid equivalent inertia constant T JS And the relationship between the disturbance intensity δ is:

[0148]

[0149] It can be seen from the above formula (21) that it can be proved again that the economic advantage of the frequency trajectory planning control FTP described in this embodiment will increase with the decrease of the disturbance intensity δ, and it also shows that the economic advantage will also increase with the grid equivalent damping coefficient K L,f , grid equivalent inertia constant T JS The equivalent damping coefficient of the power grid K L,f , grid equivalent inertia constant T JS The larger the value is, the stronger the anti-disturbance capability of the wind turbine grid-connected system itself is, and the less frequency modulation auxiliary power is required from the wind turbine. However, the virtual inertia control designed according to the maximum disturbance always outputs according to the maximum disturbance parameter even if a random small disturbance occurs in the wind turbine grid-connected system, which reduces the economic efficiency of wind turbine power generation; while the frequency trajectory planning control FTP in this embodiment regulates the system frequency according to the critical frequency trajectory, only providing the inertia damping that the system lacks, and not providing excessive frequency support, which not only ensures that the system frequency does not exceed the limit, but also reduces the degree to which the wind turbine deviates from the maximum power point, ensuring the economic efficiency of wind turbine power generation.

[0150] S4. Implementation of transient frequency modulation control

[0151] As attached Figure 4 As shown, attached Figure 4 The frequency trajectory planning implementation control block diagram in the embodiment is given in the attached Figure 4 It can be seen that after adopting the frequency trajectory planning FTP control described in this embodiment, the transient frequency modulation control system of the wind turbine rotor current measuring converter includes a phase-locked loop module, a trigger module, a speed protection module, a frequency trajectory planning module, a power control module and a current control module; wherein the phase-locked loop module is used to realize real-time detection of frequency; the trigger module and the frequency trajectory planning module are used to execute the frequency trajectory planning control operation in S2; the power control module and the current control module are used to superimpose the frequency modulation result instruction output by the frequency trajectory planning module on the basis of the original wind turbine converter module, so as to realize the control of the wind turbine to participate in frequency modulation; the speed protection module is used to protect the safe operation of the wind turbine itself and avoid the wind turbine being disconnected from the grid due to deep frequency modulation.

[0152] As attached Figure 5 As stated, attached Figure 5 A schematic diagram of the frequency trajectory partition is given in NO is the natural response curve of the system frequency when the fan does not participate in frequency modulation; f TP It is the frequency trajectory curve planned according to the system frequency threshold, which is the frequency bottom line for safe operation of the system; Figure 5 (a) is a schematic diagram of the frequency trajectory partition after the frequency deviation Δf is triggered; Figure 5 (b) is a schematic diagram of the frequency trajectory partition after the frequency change rate RoCoF is triggered; Figure 5 (c) is the frequency deviation Δf variation curve; Figure 5 (d) is the frequency change rate RoCoF curve. Since the response speed and control of the wind turbine will inevitably have errors, a certain safety margin needs to be considered in actual control, that is, a safety margin is set based on the frequency threshold specified in the grid guidelines, and a safer frequency trajectory f is planned. Re ; From the attached Figure 5 It can be seen that the frequency trajectory curve f containing the safety margin Re The frequency trajectory curve f planned according to the frequency threshold TP A buffer zone will be formed between the fan and the Re Rapid frequency response for the control target can more effectively ensure the frequency safety of the system. The constraints of the actual start value and the theoretical frequency threshold of the frequency trajectory curve are:

[0153]

[0154] Among them, F act is the actual starting value of the frequency deviation; R act is the actual starting value of the frequency change rate RoCoF; F al , R al They are the margin between the frequency threshold specified in the grid code and the actual starting value of the frequency trajectory planning.

[0155] In this embodiment, the phase-locked loop module detects the system frequency in real time. When the disturbance causes the system frequency to change, the trigger module determines the frequency deviation and the frequency change rate RoCoF according to formula (23):

[0156] (|f N -f|≥F set )∪(|df / dt|≥R set ) (twenty three)

[0157] Among them, F set is the set frequency deviation trigger value, R set It is the set frequency change rate RoCoF trigger value.

[0158] As attached Figure 4 As shown in the figure, when the detected frequency deviation or frequency change rate RoCoF exceeds the set trigger value, the frequency trajectory planning control start flag T rig1 Set to 1, and then enter the fan speed protection module.

[0159] Since the rotational kinetic energy of wind turbines is limited, the rotor speed cannot drop continuously for a long time. If the speed exceeds the safe range during frequency modulation, it will threaten the stability of the wind turbine itself and cause the wind turbine to be disconnected from the grid. Therefore, under normal circumstances, wind turbine frequency modulation needs to be equipped with a speed protection module to prevent the wind turbine speed from dropping rapidly and causing the unit to shut down.

[0160] When the fan speed meets the requirement, the fan speed flag is set to T rig2 Set to 1; T rig1 and T rig2 When both are 1, it indicates that the wind turbine has the ability to actively support the grid frequency. Then the frequency trajectory planning module performs frequency trajectory planning. Specifically, the trend of the planning curve is first determined according to RoCoF, and the frequency trajectory planning is performed according to the frequency deviation at this time. The actual frequency planning curve determined according to formula (11) is:

[0161]

[0162] The frequency planning curve f Re As the control target, the PD control method is used to control the fan frequency modulation auxiliary power P vic , and superimposed on the original MPPT working point maximum power tracking P MPPT Thus, the transient frequency can be supported.

[0163] S5. Experimental Simulation Verification

[0164] In order to prove the effectiveness and economic benefits of the wind turbine transient frequency modulation control method described in this embodiment, a semi-physical experiment based on a single wind turbine grid-connected and a simulation based on a multi-machine power system were carried out; the following simulations were respectively built in MATLAB / Simulink. Figure 6 The control model of the wind turbine grid-connected system shown in the figure and the attached Fig.13 The IEEE standard four-machine two-area model with wind turbines is shown.

[0165] The hardware circuit of the stand-alone system is simulated by RT-LAB, and the control is realized by the DSP control board; the upper computer realizes the compilation and downloading of the simulation model, and the lower computer realizes the real-time operation of the simulation model. The control parameters are modified in the DSP controller, and the results are collected by the oscilloscope. The experiment verifies the dynamic characteristics of the system frequency, frequency modulation power, and fan speed after the system undergoes a continuous transient process under different wind speeds.

[0166] S5.1. Effect of different VIC parameters on frequency response

[0167] Firstly, the frequency response of the system under different VIC parameters is verified. When the wind speed is 7.2 m / s, the dynamic response process of the proposed control strategy and VIC under continuous sudden small disturbances and large disturbances is compared and analyzed.

[0168] Small disturbance condition: at 35s, the system suddenly increases the active load by 0.1MW; large disturbance condition: at 40s, the system suddenly increases the active load by 0.3MW again.

[0169] When the fan adopts VIC control, it is difficult to design the traditional VIC parameters because the system parameters, random disturbances and other parameters are difficult to know in advance. Therefore, fixed parameters are used in simulation, K d =20e6, respectively verifying that K p When 50e6, 80e6, and 100e6, the frequency dynamic response of the wind turbine single-machine grid-connected system.

[0170] As attached Figure 7 As shown, with K p With the increase of the parameters, the lowest frequency point and frequency deviation of the system are improved; however, as shown in the attached Figure 8 As shown, when K p When it is 80e6 and 100e6, it can be found that the frequency modulation electromagnetic torque T of the system e and mechanical torque T w Obvious oscillation occurs, which causes the oscillation of the auxiliary frequency modulation power of the fan, thereby causing frequency fluctuations; at the same time, as K p As the rotor speed increases, the rotor speed decreases significantly; T e and T w The time to reach a new steady state becomes longer, and the rotor speed recovery time becomes longer, so the economic benefits of wind turbine grid connection are significantly reduced.

[0171] When the fan uses VIC to participate in frequency modulation, unreasonable control parameters may cause system frequency oscillation; at the same time, as K p As the frequency modulation auxiliary power output by the fan increases, the depth of the drop in the fan rotor speed increases, the time it takes for the electromagnetic torque and mechanical torque to reach a new steady state is longer, and the fan rotor speed recovery time is longer, which significantly reduces the grid-connected economy of the fan.

[0172] When the fan adopts the control method described in this embodiment, there is no need to perform complex control parameter design. The fan provides beneficial frequency modulation auxiliary power according to the planned frequency trajectory. While ensuring the stability of the system frequency, a deep drop in the fan rotor speed is avoided. The electromagnetic torque and mechanical torque of the fan can also reach a new balance in a short time. Compared with VIC, the FTP control in this embodiment has obvious economy.

[0173] S5.2. Effects of different wind speeds on frequency response

[0174] The experiment also verified the dynamic response of the proposed control strategy and VIC under different conditions such as low wind speed (7.2m / s), medium wind speed (9.6m / s) and high wind speed (13.2m / s) respectively when there are continuous sudden small disturbances and large disturbances; the VIC parameters simulated in this embodiment are the optimal parameters obtained through repeated experiments, but in actual working conditions, it is difficult to obtain the optimal VIC parameters due to unknown quantities such as system parameters and disturbance size.

[0175] Small disturbance condition: at 35s, the system suddenly increases the active load by 0.1MW; large disturbance condition: at 40s, the system suddenly increases the active load by 0.3MW again.

[0176] The detailed analysis of the hardware-in-the-loop experiment results is as follows: Under large interference conditions, the lowest frequency of the natural response (NR) of the wind turbine grid-connected system is 49.34 Hz, which exceeds the limit of 49.5 Hz; the control method described in this embodiment can avoid complex control parameter design; the wind turbine participates in frequency modulation according to the planned frequency trajectory, which can ensure the stability of the system frequency and avoid a significant drop in the rotor speed; T e and T w A new balance can also be reached in a short time. Compared with VIC, the control method described in this embodiment has obvious economic benefits. The dynamic response of VIC with optimal parameters and the frequency modulation performance of the proposed strategy are verified under different wind speed conditions and different interference intensities; it is worth noting that the optimal VIC parameters are obtained through repeated experiments.

[0177] like Figure 9-11 As shown in the figure, under different wind speed conditions, the available frequency modulation energy of the wind turbine is different; as the wind speed increases, the available frequency modulation energy of the wind turbine also increases; using the VIC frequency modulation strategy, when facing the same frequency event, as the frequency modulation energy increases, the frequency deviation of the system after the frequency event will decrease; however, excessive frequency support seriously sacrifices the economy of wind turbine grid-connected power generation.

[0178] When the FTP control described in this embodiment is adopted, in the face of the same frequency event, even if the wind turbine has different frequency modulation energy, the wind turbine always takes the planned frequency trajectory as the control target, and only provides the frequency modulation power that ensures that the system frequency index does not exceed the limit, so as to avoid excessive consumption of limited frequency modulation resources. The experimental results show that the proposed strategy can ensure that the frequency index of the system does not exceed the limit after the frequency event occurs, and under the same disturbance, the economic advantage of the proposed strategy becomes more obvious as the available frequency modulation energy of the wind turbine increases.

[0179] S5.3, IEEE standard 4M2A model verification

[0180] Based on the 4M2A standard model, the synchronous generator G4 is replaced by a wind turbine and the following is established: Fig.12 The simulation model of a multi-machine system including fans is shown.

[0181] like Fig.13 As shown, the system experienced two downward disturbances in succession; at 20s, the system suddenly increased by 20MW of disturbance, and the lowest point of the natural response of the system frequency was 49.83Hz, which was lower than the starting threshold of FTP, and the frequency trajectory planning module was not started. When VIC is used, in order to achieve zero-difference control of the frequency, the wind turbine needs to deeply modulate the frequency and still output frequency-modulation auxiliary power under small disturbance conditions. At 30s, the system suddenly increased by 80MW of disturbance again. When the frequency dropped to 49.7Hz, the FTP starting threshold was reached, and the frequency trajectory planning module was immediately started. The wind turbine outputted the frequency-modulation power with the planned frequency trajectory as the control target, that is, when the actual frequency dynamic curve is lower than the planned frequency trajectory, the wind turbine responds and outputs the frequency-modulation power; as shown in the attached figure Fig.13 As shown in (b), the experimental results show that in the 4M2A multi-machine system, the proposed strategy can ensure that the frequency dynamic response after the disturbance is within the buffer zone of the theoretical frequency trajectory planning curve and will not trigger the system frequency protection threshold.

[0182] The above embodiment is only one of the implementation methods that can realize the technical solution of the present invention. The scope of protection claimed by the present invention is not limited only to this embodiment, but also includes changes, replacements and other implementation methods that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention.

Claims

1. A method for transient frequency modulation control of a wind turbine, characterized in that: include: Based on the frequency dynamic response model of the wind turbine grid-connected system, the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameters is obtained; According to the relationship between the frequency threshold of the wind turbine grid-connected system and the virtual inertia control parameter, a critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is obtained; According to the critical virtual inertia control parameters for ensuring the frequency safety of the wind turbine grid-connected system, a critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and includes unknown parameters is obtained; the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and includes unknown parameters is: Among them, f TP_unknown (s) is the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters; f N (s) is the rated frequency of the wind turbine grid-connected system; s is the complex frequency; ΔP D is the step disturbance power; K L,f is the equivalent damping coefficient of the power grid; T JS is the equivalent inertia constant of the power grid; K pcr To ensure the minimum value of the droop damping coefficient in the virtual inertia control of the wind turbine grid-connected system frequency safety; K dcr To ensure the minimum value of the inertia gain coefficient in the virtual inertia control to ensure the frequency safety of the wind turbine grid-connected system; Based on the critical frequency trajectory curve that meets the frequency requirements of the wind turbine grid-connected system and contains unknown parameters, and according to the relationship between the frequency threshold of the wind turbine grid-connected system and the relay protection action threshold that meets the requirements of the power grid guidelines, a critical frequency trajectory with known parameters is obtained; the critical frequency trajectory with known parameters is: Among them, f TP_known (s) is the critical frequency trajectory with known parameters; F thr is the frequency deviation relay protection action threshold required by the grid guidelines; R thr It is the frequency change rate relay protection action threshold required by the power grid guidelines; When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, the critical frequency trajectory with known parameters is used as the frequency control target of the wind turbine grid-connected system to complete transient frequency modulation control of the wind turbine grid-connected system.

2. A method for controlling transient frequency modulation of a wind turbine according to claim 1, characterized in that: The frequency dynamic response model of the wind turbine grid-connected system is: Wherein, Δf(s) is the frequency deviation in the frequency domain after the wind turbine grid-connected system is disturbed; K d is the inertia gain coefficient in virtual inertia control; K p is the droop damping coefficient in virtual inertia control.

3. A method for controlling transient frequency modulation of a wind turbine according to claim 1, characterized in that: The relationship between the frequency threshold and the virtual inertia control parameter of the wind turbine grid-connected system is: Wherein, Δf(t) is the frequency deviation of the wind turbine grid-connected system in the time domain; t is time; RoCoF is the frequency change rate of the wind turbine grid-connected system; K d is the inertia gain coefficient in virtual inertia control; K p is the droop damping coefficient in virtual inertia control.

4. A method for controlling transient frequency modulation of a wind turbine according to claim 1, characterized in that: The critical virtual inertia control parameter for ensuring the frequency safety of the wind turbine grid-connected system is: Among them, K d is the inertia gain coefficient in virtual inertia control; K p is the droop damping coefficient in virtual inertia control.

5. A method for controlling transient frequency modulation of a wind turbine according to claim 1, characterized in that: When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, the critical frequency trajectory with known parameters is used as the frequency control target of the wind turbine grid-connected system to complete the transient frequency modulation control process of the wind turbine grid-connected system, as follows: When the actual frequency of the wind turbine grid-connected system reaches the trigger value of the critical frequency trajectory with known parameters, a general expression of the critical frequency trajectory is obtained according to the critical frequency trajectory with known parameters; According to the general expression of the critical frequency trajectory, the PD control method is adopted to keep the frequency response curve of the wind turbine grid-connected system within the safety margin range of the critical frequency trajectory with known parameters, thereby completing the transient frequency regulation control of the wind turbine grid-connected system.

6. A method for controlling transient frequency modulation of a wind turbine according to claim 5, characterized in that: The general expression of the critical frequency locus is: Among them, f TP (s) is the critical frequency trajectory; f is the actual frequency of the wind turbine grid-connected system; R is the actual frequency change rate of the wind turbine grid-connected system; sign(*) is the sign function.

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